Actinia (Gr. ray), a genus of marine radiated animals, commonly called sea anemones, from their resemblance to flowers. They are fleshy polyps, termed zoanthoria by De Blainville, and zoophyta heliantkoidea by Dr. Johnston. The body is regular and somewhat like a flower in form, more or less elongated and very contractile, enabling it to assume a great variety of shapes. It has a sac-shaped digestive apparatus, with an oval orifice, surrounded by tubular tentacles of various forms. In many species the base of the body acts as a sucker, by means of which they adhere to rocks, stones, etc, while the opposite extremity presents a disk with a central orifice. This is surrounded by tentacles either in a single row or in several rows, which act as so many arms by which the animal seizes its prey and drags it into his mouth. Its only organ, the stomach, performs almost all the functions of animal life; this has, besides its opening from the mouth, one at the bottom communicating with the general cavity of the body, which may be shut at will, making a closed sac where digestion is rapidly performed by means of active secretions.

The lower cavity is divided by folds running from the circumference toward the centre, from top to bottom of the animal, the food circulating freely among these partitions by the action of vibratory cilia on their walls. Digestion is here combined with a kind of circulation; they have no blood, no vessels, no respiration other than that effected by the currents of water in the interior, doubtless accompanied by a change of substance. The surface of the tentacles is thickly studded with microscopic vi-bratile cilia in constant motion, causing currents which bring to them their microscopic food, sweeping a space of several inches. Each tentacle is a tube, with longitudinal and circular fibres, by which it can be shortened, lengthened, and moved in all directions. Upon the tentacles are great numbers of microscopic so-called "lasso cells," each containing a long hollow thread coiled spirally within it, which can be suddenly thrust out, benumbing and arresting shrimps and small fish incautiously venturing too near these innumerable and invisible threads, and enabling the tentacles to seize and convey them to the central mouth. Similarly armed threads may also be projected from the sides of the body.

The eggs are very numerous, being in bunches on the inside of the partitions until ready to be hatched, when they escape through the stomach and mouth, or through the tentacles, into the water, giving rise to creatures like themselves, only with fewer tentacles, which are in multiples of five. The young one has only five, one in the line of the mouth and the others in two pairs laterally; so that even here there is an indication of bilateral symmetry, with definition of anterior and posterior regions. The actinia is the type of the single polyp, as distinguished from the compound coral polyps. It preys voraciously on small crabs and mollusks, and when waiting for its victims these arms are expanded like the petals of a flower, and, being tinted with very brilliant colors, they present an elegant appearance. The actinia seizes animals apparently superior in strength and bulk, engulfs them in its sac or stomach, and distending itself to a great degree, digests them rapidly, disgorging the shells and harder parts of the victim when the softer parts have been consumed. Some aetiniae are fixed, and others are free. The external tunic of the body presents both longitudinal and transverse muscular fibres, covered by a layer of skin or mucous membrane.

Nervous fibres have also been detected, and the sensibility of the animal is extreme; they contract even when a dark cloud passes over them. They may be seen at low-water, clustered upon rocks and masses of stone, which they cover, as with flowers. There they remain tenaciously adhering by their base. They are, however, capable of moving from one spot to another; and in winter they seek deeper water, where the changes of temperature do not affect them. - The sea anemone is very common on the southern shores of England and on the New England coasts; and one species (actinia Jordaica), on the shores of the Mediterranean, is esteemed a great delicacy by the Italians. The fringed actinia (metridium), the most common on the N. E. coast of North America, is, in large specimens, about 4 inches high and 3 inches across the expanded disk. They are found of various colors, pink, brown, purple, whitish, and orange, in pools among the rocks, flooded at high tide, and overhung by seaweeds. In an-thea cereus, of the British coast, there is no power of retracting the long tentacles within the body; the body is of a light chestnut color, and the numerous tentacles usually sea-green tipped with red.

It is of about the size of our fringed actinia. - See "British Sea Anemones," by Philip Henry Gosse (London, 1860), and "Coral and Coral Islands," by James D. Dana (New York, 1872).

Metridium marginatum (Fringed Actinia), expanded.

Metridium marginatum, closed.

Anthea Cereus (Opelet).